In a conventional packet communications system, upon receipt of a packet addressed to a mobile station, a base station allocates radio resources (power resources and the like) up to maximum available transmission power of the base station, and transmits the packet to the mobile station. For example, in a CDMA system, upon receipt of a packet addressed to a mobile station, a base station allocates code resources up to maximum available transmission power of the base station, and transmits the packet to the mobile station.
In the conventional packet communications system as described above, when packets arrive at a base station in a burst way, transmission power of packets in the base station significantly fluctuates temporally. Thus, fluctuation of an interference level (interference power) in a mobile station belonging to another cell becomes large.
FIG. 1 shows a schematic configuration of a conventional packet communications system in a downlink. In this drawing, it is assumed that a base station BS1 controls a cell (area) “A” and communicates with mobile stations MS1 and MS3 which are located in this cell “A”, and a base station BS2 controls a cell “B” and communicates with a mobile station MS2 which is located in this cell “B”. Description will be given below while focusing on reception situations in the mobile station MS2.
In a packet communications system performing transmission power control (TPC), based on an interference level received by a receiving station in an immediately preceding slot, transmission power of packets of a transmitting station in a next slot is determined so as to have a target SIR (signal-to-interference power ratio) which is obtained by adding a TPC margin to a required SIR of the receiving station.
For example, in the packet communications system performing the transmission power control, as shown in FIG. 2C, based on an interference level received by the mobile station MS2 in a slot 1 (see FIG. 2B), transmission power of a packet addressed to the mobile station MS2 in the base station BS2 is determined so as to have a target SIR (for example, 4 dB) which is obtained by adding a TPC margin (for example, 1 dB) to a required SIR (for example, 3 dB).
However, as shown in FIG. 2A, when a large quantity of packets arrive at the base station BS1 in a slot 2, the base station BS1 allocates code resources up to maximum available transmission power of the base station BS1. Thus, compared to total transmission power of the base station BS1 in the slot 1, total transmission power of the base station BS1 in the slot 2 is drastically increased. As a result, as shown in FIG. 2B, the interference level in the mobile station MS2 from the base station BS1 is rapidly increased in the slot 2. Therefore, as shown in FIG. 2D, in the mobile station MS2, an SIR of a received signal falls below the required SIR to cause packet reception failure.
As described above, the conventional packet communications system described above has a drawback in that, regardless of the TPC margin, the rapid increase of the interference level in the receiving station due to burstiness of packets causes the packet reception failure in many cases. Moreover, there is a drawback in that such packet reception failure deteriorates a communications quality and decreases a system capacity.